In Photo 1, you can see the heart of the Rf meter, the line section.  This is made of a cast brass body, which is silver plated. On top is the connector to the moving coil meter (via a screened cable), and in front, the entry port of the range element, more of which later.  At each end is the Rf connector to which your co-axial cable transmission line is attached.  At first glance, it appears to be a standard N-type. But looking closer shows it to have a larger than normal flange. This means that you will have to buy their connectors if you want to change to an alternative type (there are 26 different flange-mounted sockets and plugs available).Of course, it is possible to use commonly available adaptors, but this can lead to impedance matching problems. To change the flange connector, four screws need to be removed, and it’s then pulled out. The Bird ones cost about 8 times that of standard flange types. Being an in-line device, the meter can read forward and reflected power, by reversal of the element. Although not directly calibrated in VSWR, a graph is supplied with the meter where it can be read off, after both forward and reverse readings are taken.

     As well as antennas, dummy loads can be attached directly to the output side.   Photo 3 shows a few examples of Bird loads. Two types are shown: the black ones are dry loads, meaning they convect the heat away by ‘heat sink ’technique. The connectors are fixed N-type on the smaller models, but may be changed on the larger ones. Care should be taken if dismantling, as some contain a resistor made of beryllium oxide, a toxic substance.  The larger grey-paint finish unit uses mineral oil (oil-dielectric) to dissipate the heat along with traditional fins. For a given physical size the oil-filled type has a greater power dissipation. The connector is interchangeable.

The remaining photos show examples of some Bird Rf power meters. Photo 5 is of the most well-known model, the 43.Composed of line section, meter, element and case, the accuracy is quoted as +- 5% of full scale deflection (fsd).   This is the most commonly seen model, and has become an industry standard, being introduced in 1952.

Photo 7 is a more sophisticated version, model 4314B.This can read both average carrier wave and peak power levels. It uses the same elements as the 43 (fortunately!).    It does, however, require battery power to work. Accuracy is +-8% peak   +-5% CW.

Photo 8 is a multi-power range model, 4410A.This uses a different range of elements, but has a switched scale to allow many power levels to be measured on any one element.  Accuracy is +-5%(CW only).

Photo 4 shows the 4304A,a general purpose unit, with a frequency range of 25-1000MHz.It uses only one special element to cover up to 500 watt, CW. The accuracy varies with frequency, but +-6 to +-7% is typical.

Photo 6 is a rare example of a Bird crossed needle meter, model 4342.The rear of the unit has a line section that holds two elements simultaneously, one in forward direction the other in reverse.  Where the two needles cross, the VSWR can be read.

Models are also made for measuring complex waveforms such as cellular digital transmissions, and ones with built-in variable RF samplers to allow low level signals to be measured with frequency counters, oscilloscopes, etc.

 Buying second-hand: the pitfalls.

      Like any moving coil metered instrument, they are susceptible to physical shock. This usually manifests itself in a ‘balance’ problem. Hold the meter both vertical and horizontal. The needle should stay at zero if all is well.  If it doesn’t, this will be an expensive repair, with replacement of the entire meter assembly being the only option. The pivots and hairsprings in the movement are prone to damage by sudden shock

     With more sophisticated models, the question of calibration becomes paramount, and unless you have access to accurate RF sources, the cost involved in having a meter re-calibrated may make the purchase price unrealistic. For calibration purposes, Bird manufacture RF calorimeters with accuracies of +-1.25%.However, you are unlikely to see one of these at a rally (and no, I don’t have one!).

The elements are fairly robust, having ptfe insulated backs and gold-plated bodies. Too much RF can damage them, but the only way to find out if it works is to try it in a known working meter.

In view of the cost of new Japanese/Chinese made wattmeter/vswr units and dummy loads, second hand Bird equipment can be bought at reasonable prices. Search the web for the company’s site, so you know what each item can be expected to do.  Finally, as in all purchasing, caveat emptor

                   73

                   Steve  M0OXR